1. Field of the Invention
This invention relates to arrangements for providing protection to personnel servicing a distribution bus. In particular, it relates to apparatus and a method of grounding the distribution bus using a circuit breaker.
2. Background Information
It has been common to provide protection for workers servicing a distribution bus to include an earthing switch between the circuit breaker connecting the distribution bus to a main bus so that the distribution bus may be grounded. In some installations, it is now known to place the earthing switch on the line or main bus side of the circuit breaker. However, the earthing switch and not the circuit breaker is used to complete the connection to ground.
The earthing switch is used during servicing to remove voltage from the distribution bus. However, there could be a back feed to the distribution bus. For example the distribution bus could be double fed, there could be a back fed transformer on the distribution bus, or there could be other errors.
Current requirements call for the earthing switch to be able to close in on two faults and still operate. However, under a new requirement the earthing switch will have to close in on five faults and still operate.
Closing in on a fault causes an arc. The arc in turn causes melting of the contact material which can result in welding of the contacts. If the contacts are not opened while the metal is still fluid, a rough surface is produced. The voltage concentrations caused by the spikes on the now rough surface result in an even earlier striking of the arc the next time and can lead to permanent welding of the contacts. The copper air blade switches normally used as earthing switches can not satisfy the five fault closing requirement. They will weld or be destroyed. It is too expensive to develop a specialized selector switch which could close in on a fault five times and still function.
Circuit breakers are made to close in on high currents, but even they suffer deterioration of the contacts over multiple high current closings. This is true even on vacuum interrupters which can stand much higher currents than air blade switches. It is common practice with circuit breakers to trip immediately upon closing into a fault. Thus, the metal is still molten and forms a smooth surface after contact separation. It is also common for a circuit breaker to have an auxiliary switch which provides an indication of the open/close state of the main contacts. In some cases, these auxiliary breaker switches have a time delay. The time delay is used to prevent tripping of the circuit breaker when closing to allow other breakers down stream to respond to a fault upon closing of the breaker.
There is room for improvement in providing protection for personnel servicing a load in an electric power distribution system.
This invention is directed to protecting personnel servicing an electric power system through the use of a ground selector switch and a circuit breaker with intelligence. The ground selector switch in connected in series with the circuit breaker between the main bus and the distribution bus on the main bus side of the circuit breaker. The ground selector switch is moved to the ground position with the main contacts of the circuit breaker open. The main contacts of the circuit breaker are then closed. If the circuit breaker is closed in on a current, an unsafe condition exists and the main contacts are opened without any intentional delay. On the other hand, if the distribution bus has been connected to ground, that is the ground selector switch is in the ground position and the circuit breaker has been closed for a selected period of time before current flows, the main contacts of the circuit breaker are not opened. This is done to maintain the ground state of the distribution bus previously established. Under these conditions, other devices in the system should respond to the current.
More particularly, the invention is directed to a method of providing protection for servicing a distribution bus connected to a main bus through main contacts of a circuit breaker comprising the steps of: connecting the circuit breaker to ground between the main contacts and the main bus, closing the main contacts of the circuit breaker, and opening the main contacts without delay when current flows through the main contacts within a selected time period after the closing of the main contacts. The method further includes blocking opening of the main contacts if current begins to flow through the main contacts only after the selected time period after the main contacts have been closed with the circuit breaker connected to ground between the main contacts and the main bus. In this latter case, a warning signal is provided indicating the no trip condition.
The invention is also directed to apparatus for providing protection when servicing a distribution bus fed from a main bus comprising a circuit breaker having main contacts and connected to the distribution bus. An earthing switch is connected to the circuit breaker and to the main bus. The earthing switch has a first condition in which the circuit breaker is connected to the main bus and a second condition in which it is connected to ground. The circuit breaker includes a protection circuit, which is operative with the earthing switch in the second condition, that is connected to ground, to open the main contacts of the circuit breaker only if current flows through the main contacts within a selected time period after the main contacts close. The protection circuit can include an auxiliary switch having a first condition when the main contacts are open and switching to a second condition the selected time after the main contacts close. The protection circuit further includes a current detector detecting current flowing through the closed main contacts and a trip circuit responsive to the earthing switch being in the second condition with the auxiliary switch in the first condition, and the detection of current by the current detector to generate a trip signal without delay. The circuit breaker also includes an operating mechanism for opening the main contacts in response to the trip signal. Preferably, the trip circuit includes means periodically sampling the condition of the earthing switch, the condition of the auxiliary switch and the current detected by the current detector and generating a trip signal only after a predetermined number of samples in which the earth switch is in the second condition, the auxiliary switch is in the first condition and current is detected. The predetermined number of samples is selected so that it occurs in less than one cycle of the current and preferably in about one half cycle. The trip circuit can be operated by the current detected by the current detector.